DISCUSSION OF REPAIR OPTIONS FOR MASONRY STRUCTURES…nigelcopsey.com/reports/malton/malton_discussion.pdf · DISCUSSION OF REPAIR OPTIONS FOR MASONRY STRUCTURES, MALTON, NORTH YORKSHIRE

DISCUSSION OF REPAIR OPTIONS FOR MASONRY STRUCTURES,MALTON, NORTH YORKSHIRE

NIGEL COPSEYOCTOBER 2004

the earth stone and lime companybuilding conservation consultancy and practice

As a company, we subscribe broadly to the principles and objectives ofthe Society for the Protection of Ancient Buildings (www.spab.org.uk) andto the ideals of its founder, William Morris. We aspire to a thoroughgoingand articulate day to day ethical practice in accordance with internationalconservation treaties.

We place a strong emphasis upon the promotion of open, honest andunhierarchical dialogue between all those involved in a project, as well asengaging with the craft, history and cultural significance of the buildingor monument itself.

We believe that there are no circumstances in which the use of ordinaryportland cement mortars may be either necessary or justified in thecontext of a structure not originally constructed using these materials.

We use lime mortars and traditional materials whose performance hasbeen tested over centuries of use in all climates and conditions.

We believe that the skilled and informed use of traditional materials suchas wood, stone, brick, lime and earth is not only good for the buildings ofwhich they are a part, but good for the environment as a whole and for ahealthy life in general.

[email protected] www.nigelcopsey.com

Passion and expertise in the care and repair of old buildings

This document represents a contribution to the discussion of issuesrelevant to the effective care and repair of masonry structures in the townof Malton, North Yorkshire. It has been written in the context ofconservation and repair works carried out by myself for the FitzwilliamEstate, and includes the report upon these. My approach to the works tothe Talbot Coach House wall and to the lower level of the adjoining househas been to execute these in a manner which provides practical examplesof available repair options and which will allow a longer-term assessmentof these. I have also taken the opportunity to discreetly trial a variety oflime mortar designs, the relative performance of which may be assessedand compared overtime. It is my hope that this report/discussiondocument will contribute to the necessary evolution of repair strategieswithin the specific context of Malton in keeping with more generalconservation strategies.

The objective conditions would seem to exist for the establishmentlocally of a well-articulated and thoroughgoing policy of care and repairwithin the town, to the enhancement of its aesthetic and commercialenvironment and to the well-being of its historic fabric.

The majority of the historic building stock of the town is in singleownership, and the owner is committed to the proper conservation of itsbuildings, utilising appropriate traditional materials (particularly lime-mortars) in a proper ethical and practical context.

Ryedale District Council, through the offices of its Conservation andListed Buildings Officer, is supportive of this approach, and is astakeholder, along with North Yorkshire County Council and EnglishHeritage, in the Heritage Economic Regeneration Scheme.

Furthermore, the skills and the understanding necessary to carry out bestpractice conservation repair exist locally. Education and practical trainingis readily available and may be accessed easily and affordably by thoselocal building contractors not already familiar with the proper use oftraditional materials.

Experience in the South-West, and particularly in South Somersetdemonstrates that a confident, dynamic and pro-active local authorityconservation policy, coupled with a plentiful supply of specialist practicalbuilding conservation expertise locally leads to a much-increasedawareness and take-up of (old) new skills on the part of local building

contractors formerly wedded to inappropriate and destructive ‘cementtechnology’.

Good quality lime, whether putty or hydraulic, is readily available within a20 mile radius of the town.

Malton enjoys, therefore, a perhaps unique opportunity to become aregional exemplar of good and effective building conservation practice.

This document should be read in conjunction with Seamus Hanna’s‘Report On The Approach To Preservation Of Stonework On VernacularBuildings In Kirkbymoorside, Malton and Pickering, North Yorkshire’ forthe English Heritage Building Conservation and Research Team.

Malton is constructed mainly of brick, sandstone and of two distinct typesof limestone. Stone buildings are frequently built of a mixture oflimestone and sandstone. Its builders commonly used plinths of localsandstone below mainly limestone walls, although free-standingboundary walls and houses are as often built entirely of sandstone. Themajority of stone walls in the town have lower courses of sandstone andthis presents notable patterns of decay, to a greater or lesser extent,throughout the town.

Lower Castlegate

The perceived aesthetic and structural problems associated with thisdecay have been addressed in a variety of ways over the years. There areexamples of refacing of decayed wall-bases with brick, with new stoneand with render. The renders used have been almost universally basedupon ordinary Portland, or ’roman’ cements. These are equally universallydetaching and, if not loose, they are hollow and cracked. This materialallows moisture into the already decayed substrate whilst at the sametime preventing its ready egress. It introduces and traps damaging salts.It most certainly does nothing to enhance the structural qualities of thestone behind. There is a spiral of decay at work here, and certainly notthe considered management of gradual decay as aspired to by EnglishHeritage.

The majority of stonework in the town, whether to houses or boundarywalls, has been repointed with ordinary Portland cement-based mortar,which is damaging to any masonry, and especially so to relatively softand/or decaying stone or brick. Not only the material, but the method ofthis repointing work is damaging and more likely to promote than toinhibit decay: more recent pointing is most commonly of a ribbon/straptype, standing proud of the stone substrate, presenting a matrix ofledges to collect water before channelling it into the building. Thismoisture then finds egress impossible except via the stones themselves,

and this more slowly than would be the case via soft lime mortar joints,promoting frost damage and the concentration of salts which willcrystallise in drying conditions just behind the face of the stone,progressively bursting it away.

Throughout the town, the enhanced decay of lower courses of sandstoneto a height of between 2 and 3 feet is notable. Plinths of other materials,whether brick or limestone have not decayed to anything like the sameextent. Such decay as there is to brick or limestone plinths may beconfidently ascribed primarily to the effects of ill-advised opc mortarrepointing and to acid attack from vehicle exhausts.

Typical roadside plinth decay, Yorkersgate

The decay to sandstone plinths is not universal, however. With hindsight,of course, it is possible to assert that the stone was ill-chosen for itspurpose. However, many of the most decayed plinths are of large bed-height and of large dimension generally. In the case of the Talbot HotelCoach House Wall, it is at least possible that these stones were recycledfrom the mediaeval town wall, which stood close by. It is quite possiblethat the Old Lodge Wall is either a remnant of the castle precinct, or isconstructed of stones recycled from the castle itself. These stones may

have been out of the ground for longer than most, therefore.

What would seem beyond doubt is that the decay of sandstone plinthsthroughout the town is particularly associated with damage by salts.Added to this is a general vulnerability of calcareous sandstone to decayinduced by acids, particularly form nitrogen compounds produced as aresult of the burning of fossil fuels, whether coal, gas or petroleumproducts such as petrol and diesel. Ordinary Portland cement is rich insulphur, and the repointing of sandstones with opc mortar will haveaccelerated the consequences of acid and salt attack.

The general source of salts within masonry structures is from the ground.Very many of the decaying sandstone courses are not only on the ground,but retain ground, and the abundance of salts is therefore inevitable.Many of the plinths carry limestone masonry and are therefore at least intheory vulnerable to decay mechanisms associated with this association(see Hanna). I am not convinced, however, that this is a significant decaymechanism in this instance.

There are many plinths of like sandstone that present no decaycomparable to that of the Coach House Wall, for example. Similarly, thereare many sandstone plinths that are vulnerable to ground salt migrationthat do not display any comparable decay, or, indeed, any more decaythan sandstones at higher levels in the building, well-away from thereach of ground-salt migrations. There are also buildings in the town thatare built with a mixture of local lime and sandstone, the church being aprime example. Sandstone below limestone on the tower of the Church ofSt Leonard’s with St Marys shows no different a pattern of erosion tosandstone above or well away from this limestone.

Mixed lime and sandstone, southwall, tower, St Leonards Church

West wall of same

In situations where a sandstone is vulnerable to calcium carbonate, decaytends to be particularly evident in the sandstone immediately below thelimestone. Such vulnerability manifests in the arrisses of sandstone at thejoints also, since, of course, the original bedding and pointing mortarsare of lime mortar. The combination of lime and limestone dust in amortar creates a calcium carbonate material chemically very similar tolimestone. It is a characteristic of historic mortars locally that theycontain a high proportion of limestone dust. If the local sandstone wasvulnerable to decay by proximity to calcium carbonate, then one wouldexpect to see a pattern of significant arriss decay across the whole rangeof walls and buildings in the town, as is the case in Worcestershire, forexample, with or without the presence of limestone in the building. Thisis clearly not the case. The majority of arriss decay in the town isassociated with the inappropriate use of ordinary Portland cement-basedrepointing mortars and more gradual powdering associated withatmospheric sulphur and carbon compounds.

The sandstones of the Vale of Pickering are Calcareous sandstones ofvarying silica content (the silica content of samples examined by JeffersonConsulting varied from only 27% in stone from the Old Lodge Wall,

Malton, to 57% in stone from Park Street, Pickering). As suggested bytheir yellow colour, they contain iron. The primary binder of Calcareoussandstone, however, is calcium carbonate. This makes them vulnerable todecay by airborne acids, in similar ways to limestones, although thepatterns of decay in limestone and sandstone will manifest differently. Itmakes it highly unlikely that the proximity of calcium carbonate in theform of limestone is a likely primary cause of decay, since localsandstones are themselves rich in calcium carbonate. The relatively lowcalcite content of the Jurassic sandstone around Whitby, and the resultsof tests on historic sandstone in the Vale of Pickering suggest that thereis a decreasing silica content, and therefore an increasing proportion ofcalcite (CaCO3), north to south, with the sandstones of the Malton areahaving a high CaCO3 content.

It would seem clear to me that the high levels of degradation tosandstone plinths in Malton is associated primarily with splash fromrock-salted roads, with associated decay caused by high concentrationsof vehicle exhaust gases at lower levels. Wherever plinths are defendedby distance or elevation from road-splash, or less heavy concentrationsof vehicle exhaust emissions, they show little differential decay to therest of the elevation. Wherever they are vulnerable, and particularly onthe roads that have the greatest volume of traffic travelling at thegreatest speed, there is significant decay. The regular and frequentsaturation by salt-laden water of walls that have been pointed with hardand impervious opc-based mortars has disproportionately exposed thevulnerability of the local sandstone to decay, incorporating other decaymechanisms such as frost in a devastating alliance.

Old Lodge wall, north (road-) side

Inspection of patterns of decay to the Old Lodge Wall shows that thedecay to the road-side of the wall is much more extensive, and of adifferent pattern to that on the Hotel side. The inside face has relativelyeven decay, with none of the undercutting particularly at lower levelsevident on the outside face. Given that the inside is the southerlyelevation, one would expect more erosion on this face, due to its greatervulnerability to the damaging effects of freeze/thaw cycles during thewinter. The Estate Offices, remnants, along with the Old Lodge Wall of theJacobean house, which are set back from the road and the plinths ofwhich are at a higher level than the road show similar patterns of decayto the inside elevation of the Lodge wall, and none of the undercuttingevident to the outside of the Lodge wall.

Old Lodge wall, south side

Buildings in the estate yard, well away from road splash and heavyconcentrations of exhaust gases show even and gradual erosion acrosstheir entire elevations, despite having been repointed with hard cementmortars.

Outbuilding, estate yard

Regarding decay associated with the burning of fossil fuels, this issummarised succinctly by Honeyborne:

“ A small minority of sandstones used for building in the UK arecemented by calcite (calcareous sandstone)…These sandstones weathermore severely than limestones in regions of high air pollution becausethe dissolving of a small amount of calcite will release many sand grains.Where the stone is heavily rain-washed, the surface will steadily powderaway. When the stone is not heavily rain-washed, the acids attack someof the calcite converting it to the more soluble gypsum. Some of thegypsum is drawn towards the surface in solution and redeposited as themoisture dries out again. This produces a weakened layer roughly 4mmbelow the surface of the stone which blocks the pores of the surfacelayer. As the temperature varies, the unrestrained gypsum expands orcontracts about 1.7 times as much as sandstone. Changes of temperaturewill therefore create stresses that tend to break the gypsum-rich layerfrom the underlying sandstone.” (Weathering and Decay of Masonry, Ch

7 of Ashurst and Dimes’ Conservation of Building and DecorativeStones, Vol 1, p157)

Wind will amplify this cycle of decay, creating patterns of greater erosionto the more exposed areas of a structure, even to the more exposed partsof individual stones (as may be seen on the Vanbrugh Arch and the piersof the Old Coach House Wall). Wind is not in itself the cause of the decay,insofar as the erosion is not caused by windborne abrasive material.

I would reinforce Hanna’s assertion that the use of silane-based stoneconsolidants is uncalled for in the context of Malton. The Society for theProtection of Ancient Buildings maintains a moratorium on the use ofsuch consolidants and research by Dr Clifford Price suggests that thelikely depth of penetration is actually minimal. In circumstances wheremoisture is absorbed into the stone from the ground below or behind thestones, then a consolidated (evaporating) face would likely acceleraterather than slow decay of the stone. The use of water repellants shouldnever be allowed upon porous masonry. These will lead to a rapiddegradation of the faces of the stones, compounding the problem theyare offered as solution to.

The earlier use of lime mortars for repointing and, even, for renderingwork would have reduced the impact of road salt damage considerably.It would also have reduced the vulnerability of the masonry to thesulphur-soluble gypsum cycle. Lime mortars allow ready breatheability aswell as providing sacrificial poultices for salts. I would hazard that thelevels of decay to these stones has accelerated dramatically since theintroduction of road-salting, Portland cement and the increase in car andlorry volumes.

Also associated with road traffic is the heavy sulphation of limestone inwalls alongside main roads in the town. Yorkersgate and Castlegate areused by around 100 HGVs daily. Traffic is frequently slow-moving onboth roads. The buildings in Castlegate are heavily discoloured by dieselparticulates, which dust their entire elevations. This is aestheticallydispleasing and unhealthy; the sulphation-crusting of limestones causestheir progressive decay. The Coach House wall has thick crusts in manyplaces; where it does not it is because these crusts have blistered anddetached, taking significant volumes of stone with them. Thesesulphation crusts will re-establish over time. The Vanbrugh Arch isheavily soiled with such crusts and has suffered significant decay andloss of detail at least partly as a result.

There are three main strategic decisions that would have greatest positiveimpact upon the levels and rate of decay in Malton:

1) Use only lime mortars ( which means mortars with ZERO ordinaryPortland cement content) in repair and repointing works, and promote apolicy of removal of opc repointing work as a matter of priority;

2) Significantly reduce the impact of road-salting. It may not be possibleto stop salting roads (although climate-change may reduce the necessityover time). However, a significantly reduced speed limit through the townwould reduce the range of splash, as would regular maintenance ofdrains (many of which are currently full of leaves). Any reduction in trafficvolumes would be of benefit to the historic fabric of the town.

3) Take steps to remove all but essential HGV traffic through the town.The existing by-pass does not do this, or, at least renders it necessaryfor HGVs heading into Norton to pass along Yorkersgate and Castlegate.

REPAIR OPTIONS

Although the sandstone locally is prone to significant decay in certainsituations, it retains good compressive strength and structural integritybehind the loose or powdering surface. Generally, the stones that showthe most pronounced decay are large and extend a long way back intothe wall.

As stated by Seamus Hanna, the presumption of any repair strategyshould be intervention to the minimum necessary to slow the rate ofdecay as far as possible whilst at the same time maintaining overallstructural integrity of the wall or building. Extensive stone replacementwould be hugely expensive, even if a compatible stone could be found.The basic soundness of the stones that might be candidates forreplacement means that any full-depth replacement would be hard-going, time-consuming and the brute force involved would carry asignificant risk of disrupting the surrounding masonry. Facing up theplinths with a partially structural facing of new stone would be to invitean enhanced risk of structural disruption or failure in the future andcannot be justified on the grounds of ‘taste’ alone. New stones alter the

overall character of a weathered and eroded wall considerably, especiallywhere the weathering takes the form of a rounding of the old stone to itsjoints. Whilst the use of pitch-faced replacement stones reduces thiseffect, it is by no means certain that the original profile of the masonrywas pitch-faced. It is also common for stone-facings to lack adequateback-filling of mortar, even with the best intentions of the mason. It isdifficult for them ever to receive normal loading from the masonry above(unless this subsides onto them) and there is always the possibility,therefore, of their ‘popping out’ in time. There will be occasions when theinsertion of new stone will be considered necessary or desirable. Myfeeling, however, is that in general this offers the least satisfactoryoption. It works best when the stone around it is only minimallyweathered. Using it in this circumstance begs the question as to whetheror not it was structurally necessary. There are decayed plinths in Maltonwhere the decayed stone is itself a facing to the original masonry, usingsandstone from without the Vale of Pickering.

Whilst gritstone from West Yorkshire has fared very well (almost too well,since it has reduced very little and feels quite at odds with the localmaterials), replacement stones from the Whitby area would seem to havedecayed even more readily at lower levels than the local sandstones. Abetter case for replacing these replacements with further facing stonescan be made than for facing up original material in this way.

There are relatively few sandstone plinth stones that have been eroded toa depth such as to undermine the stability of the wall, or even ofindividual stones above. The sandstone plinths were often stepped outsome 4/ 5 inches from the wall above. The centre of gravity of the wallsabove continue to bear upon sound stone: they are still fit for purpose.Patterns of decay vary one to another, resulting in a visual chaos ofdecaying profiles. The initial, unconsidered perception is that these wallsare more precarious than they are in fact.

The erosion and decay is progressive, however, and there will come apoint when the structural integrity of otherwise sound walls will becompromised. This is becoming the case in parts of the town, wherewalls were built in a single plane, without projecting plinths.

Tidying up the reading of these walls and slowing the processes of decaywould be compatible and complimentary objectives.

It is my opinion that this could best be done with a combination of repairtechniques based upon the use of lime mortars. The objective would be

consolidation of the substrate using relatively discreet but honest repairmethods.

Defrassing of loose material and the removal of opc mortar should befollowed by an assessment, stone by stone, of the risk factors in furtherdecay. Any significant areas of undercutting might be filled with a stoneor clay tile repair. This may involve a limited amount of chiselling away oforiginal material to provide a flat base or a key for tiles of variable depth(the introduction of stone facings requires the removal of far more soundmaterial and is more aggressively disruptive of the surrounding wall). Tilerepairs are built up in thin layers, allowing for reliable consolidation ofthe mortar placed around and behind the tile. They are flexible, and maybe contoured to blend in profile with the profiles of adjacent stones.Alternatively, they may provide a good key or termination for mortar orrender repairs to less heavily decayed adjacent stones. The use ofhandmade clay tiles is considered by some to be a little obtrusive.However, it is my conviction that they effectively demarcate or signifyareas of repair in a transparent but generally satisfactory manner. Theyare generally far less obtrusive in practice than new stone and areaesthetically pleasing in themselves. They are relatively soft and flexible.

There is an increasing tendency in stone conservation practice tosubstitute stone for clay tiles. These are used at 1 inch thickness, orthinner if practicable. It is difficult to saw sandstone reliably at less thanthis thickness. A hand-made clay tile has a beautifully varied butrelatively straight edge, with a lot of character. Stone tiles have a sawnprofile, which has much less inherent character, but which is not alwaysunsuitable.

An advantage offered by tile repair is the provision of multiple lime jointswhich will enhance the breatheability of the wall-face, speeding thedrying and evacuation of salts. The lime face may decay sacrificially, butis readily repointed should this occur.

In association with lime-pointing and tile repair, discreet consolidation ofthe stonework should be carried out using lime mortar. Voids and water-traps should be filled to maximise the shedding of water. Deeply erodedbeds within certain stones may be filled out with a lime mortar of similarcolour to the stone. Such ‘dentistry’ repair is a common approach in theSouth-West of England.

All of these procedures need to be executed with sensitivity and with theoverall aim of maximising the longevity of the fabric withoutcompromising its character.

THE TALBOT COACH HOUSE WALL.

Works to the Coach House wall were specified by Julie Smith of Lightlyand Lightly to include repointing as necessary, the replacement ofmissing stones, the rebedding of coping stones and the rebuild asnecessary of pillars that once held iron railings.

The wall is a retaining wall to its full height.

Repointing work was carried out in four slightly different mortars,offering an ongoing opportunity to monitor their relative performance. Afifth variation will be used for the repointing of the especially fine jointsof the Vanbrugh arch. All mortars were lime mortars with the addition ofno ordinary Portland cement.

At the same time, repair works were carried out to the lower section ofthe house that adjoins the site.

Repairs to the house wall included the replacement of 6 no. severelyeroded stones (themselves refacings), removal of hard cement strappointing and its replacement with lime mortar, as well as the limerendering of more heavily eroded stone facings.

Eroded refacing stone, Yorkersgate

The section of wall between the house and its yard gateway had a badly

eroded base.

Eroded wall base, Yorkersgate

One limestone refacing had lost all compressive strength and wasremoved. Stones above the base course were deeply eroded on theirlower sections. The quoin had been previously rebuilt using small stonesand a very hard opc mortar. The defective stone was replaced with a 4”deep facing of sandstone ashlar from the Blue Bank Quarry betweenSleights and Whitby. The erosion to the stone above was repaired usingsandstone tiles; the small quoin section was reformed with handmadeclay tile. The deep erosion to the base stones to either side of the newstone was built up with lime mortar ( with inclusions of clay tile pressedin to assist curing and to help key subsequent render coats. Thesestones, whilst deeply eroded, remained fit for purpose. Their effectiverefacing would have involved heavy hammering which might have undulydisturbed the wall, which is high but only some 4 feet long. The finalcoats of lime render were made up of sharp sand and Ancaster limestonedust at a 50/50 ratio, mixed 3 parts aggregate to 1 part Hl2 naturalhydraulic lime from Tout Quarry, Somerset. Although this is a 3.5 NHL, itdelivers a significantly softer lime mortar than St Astier 3.5. It is also acream colour, which is an advantage in mortar matching.

clay, stone tile and render repair to the above

The intention of the lime render is to provide a sacrificial face to the baseof a section of the wall particularly vulnerable to the effects of vehiclewashed salts. The end of the adjoining building forms a corner and awater trap at the base of the wall.

One further clay tile repair was carried out further up the wall, wheredifferential erosion in one stone had led to a severe horizontal undercut.The purpose of the tile repair was to pre-empt the detachment of theoverhanging section of stone.

before repair

after repair

The limestone masonry was essentially sound and evenly eroded. Oddstones that had been laid face-bedded displayed somewhat deepererosion.

Erosion to the freestanding piers above the wall was generally more

severe. The piers were constructed of the same local fine-grainedlimestone as the upper parts of the wall and the Vanbrugh Arch, but withgritstone capstones, probably from the Whitby area, possibly from WestYorkshire. These have weathered only minimally and may not have beenoriginal. The stones of the piers were originaly masoned ashlar withprecisely tooled margins. For the most part, this detail has been lost toerosion. The main mechanism of erosion has been wind, along withrepointing in the past with a hard and dense ordinary Portland cementmortar, which has kept the stones wetter than would a lime mortar andmore vulnerable to the actions of frost and wind. Upper courses of stonehad been rebedded in opc mortar also, and capstones rebedded in thesame.

repointing; rebuilt pier.

The hard mortar had come loose long-since and was providing noadhesion between capstones or between the stones themselves.

The piers are none of them of exactly the same dimension, and neverwere. Those between sections 1 and 2 and between 2 and 3 were slightlyrectangular on plan, the longest side running parallel with the wall.Others were square on plan. The pier caps had been made to measure.All piers have settled with the settlement of the wall beneath and none ofthem are in the same orientation as another. This differential settlement

is part of the character of any old structure.

The capstones were lifted off and the piers dismantled only insofar asindividual stones were loose. The masonry of the pier cap betweensections 4 and 5 was universally loose and had been disrupted by ivyroots in the past, its joints had accumulated organic matter to a fulldepth. This pier was dismantled to the top of the wall beneath and thestones set aside to ensure that each course returned in the same lay-out.All hard cement mortar was removed from faces and beds. The pier wasrebuilt using a coarse lime mortar.

The patterns of wind-enhanced erosion made it very difficult to rebuildthe piers reliably using a level; reliable reference points were hard to find.It was possible to align patterns of weathering between the lower edge ofone stone and the upper of another. The objective was to follow thesettlement of the base and to work mainly by eye, using a level mainly onthe top beds. Taking the lead from the existing lie of the wall/soundsection of the pier cap was essential to maintaining the piers’contribution to the overall character of the wall. It would have been amistake, and more obviously contrived, to rebuild the piers absolutelyplumb and true in themselves when the wall of which they are a part isnone of these things.

The top two courses of the piers between sections 2 and 3 and betweensections 3 and 4 were dismantled and the still sound courses belowconsolidated as necessary. It is notable that the main areas of disruptionwere those that had been rebuilt in the past using hard cement mortars.This had been unable to accommodate movement from below and had

left the stones loose, held in place only by the mass of the sandstonecaps. The upper section of the pier between section 1and 2 was veryloose; a corner had been broken from the pier cap, suggesting a recentimpact, since the damage is not evident on survey photographs. All pierswere reassembled using a course lime mortar and were repointed usingthe same.

The sandstone plinth had been built with bed-joints generally no widerthan 5mm. Perpendicular joints tended to be wider than this, and erosionhad made them appear wider still. Pointing works kept to the originalwidths where appropriate. Elsewhere, pointing was laid in a little wider, totake account of patterns of erosion and to provide weatherings. Wherelarger sections of stone face were developing sheer cracks and wherethere were clear water traps, these were filled with lime mortar to provideweatherings. Large voids were packed with lime mortar and fragments ofstone or tile and faced up with lime mortar.

The pointing mixes used were as follows:

To the house wall : 2 parts plastering sand (Walkers, York); 1 partlimestone dust (Ryedale Conservation, Terrington); 1 part hl2, Somerset(supplied by Walkers, York);

To the lower section ( Section 5 , Smith specification) : 1 part plasteringsand; 3 parts sharp sand (Travis Perkins); 2 limestone dust; 1 part puttylime ( Chalk Hill, supplied by Ryedale conservation); 1 part hl2;

To sections 3 and 4 : 2 parts plastering sand; 3 parts sharp sand; 1 partputty lime; 1 part hl2;

To sections 1 and 2: 1 part building (soft) sand, 1 part sharp sand (bothTravis Perkins), 1 part limestone dust, one part hl2.

To pier caps: 2 parts sharp sand; 1 part limestone dust; 1 part hl2.

The repointing mix for the fine joints of the Vanbrugh Arch were as forthe house wall : 2 parts plastering sand; 1 part limestone dust; I part hl2.

General remarks:

In general, I would not recommend the use or inclusion of soft buildingsand in lime mortar mixes. It is not well graded, being comprised ofgrains all of the same size. It has no compressive strength. It is used inthe building trade in the UK with ordinary Portland cement. Such mortarsderive their compressive strength from the binder (opc). (In the USA,masons continue to use well-graded sharp sands even with cement,being more connected to craft tradition). Workers more familiar withusing opc will tend to add too much soft sand to their mixes, so used arethey to its use.

Lime mortars derive compressive strength from the aggregates used. It isessential, therefore, that aggregates are well-graded. Sharp andplastering sands are well-graded. Plastering sand is finer than sharpsand. Both resist shrinkage very well. The use or inclusion of soft sandpromotes shrinkage.

The addition of stone dust may also promote shrinkage, unless it toocontains a range of granule size. 5mm to dust is appropriate.

Initial slight shrinkage is not in itself a problem with lime mortars, aslong as proper aftercare is carried out, and the mortars arerecompressed, beaten back and moistened whilst still pliable. It ispreferable to avoid shrinkage in the first place, however.

Abundant wetting of the substrate before repointing is a fair guarantor oflow shrinkage and good adhesion of the mortar to the masonry substrate.

Mortars should hold the minimum amount of water necessary to deliverworkability. Over-wet mortars will stain the stonework and be more likelyto shrink. They will also be weaker. If the walls have been well-wateredbefore work, and the new pointing is protected with regularly dampenedhessian/burlap, moisture will diffuse through the mortar, promoting agradual set, whether by carbonation or chemical reaction, or both.

The use of putty lime, with the addition of pozzalans is to be encouraged,as also the use of Natural Hydraulic Lime.

The gauging of putty lime mortars with up to 50% by volume of NHL isperfectly acceptable. The Smeaton Project found that a mortar made witha lime content 50/50 putty to NHL 3.5 produced a mortar with propertiesof vapour permeability and compressive strength similar to that of a puttylime mortar. The use of NHL, however, speeds the initial setting time andcan offer advantages in terms of work speed and can reduce thetimespan of a fresh mortar’s vulnerability to frost damage.

It is my experience that St Astier 3.5 delivers a mortar that may be toohard for use in repairs to soft and decaying masonry. There is a variabilityeven in St Astier 2.0 that can at times see it being as hard in fact as a 5.0NHL.

If St Astier NHL is used in the context of work within Malton, I wouldrecommend that only 2.0 be used and that, even then, this should begauged with 40-50% lime putty.

Hl2, an NHL made in Charlton Mackerel, Somerset from Blue Liaslimestone, although rated 3.5 delivers a mortar that is softer and morevapour permeable than that achieved with St Astier 3.5. It is in myopinion the more appropriate NHL for use in the Malton context. It maybe used with or without the addition of putty lime.

For more general guidance on good practice with lime mortars, seeattached texts below.

THE VANBRUGH ARCH

The Vanbrugh Arch provided access from the Talbot Hunting Lodge tothe stables. It is a composite structure of stone and brick, although thestone archway is a self-supporting construction. It gives access to aclamp brick-built passage and gritstone paved stair.

The Arch is exposed to the prevailing wind and is effectively a windtunnel. This has led to uneven patterns of erosion.

The arch is built mainly of local fine-grained limestone from quarries atHildenley, near Malton, with a Whitby area sandstone plinth course. It hasoccasional insertions of local sandstone. All but one of these would havebeen unseen when the arch was built, but have been exposed by erosionof the limestone. One large block of local sandstone is structural andforms part of the rusticated jamb to the lower side. It is badly eroded toits face. This erosion is progressive and is both more extensive and morerapid than the ongoing erosion of the limestone. Its replacementrepresents a dilemma because the erosion of the limestone ashlar issignificant. Its replacement with a piece of new limestone, or like-for-likewould jar visually. It would be both difficult and affected to seek toartificially reproduce the pattern of erosion elsewhere in the archway in anew stone. No historic source for this limestone has yet been identifiedlocally, although it bears a great similarity of appearance andcharacteristics (and I would hazard, geology) to base-bed Portland stone.It smells exactly as Portland stone does upon impact with a chisel, andresponds to the tool in a very similar fashion. It is a different limestone tothe dolomitic limestones used in York and sourced from Tadcaster andCadeby. These have a different geology and quite distinct patterns oferosion, and should not be used in replacement work in Malton.

The arch stones were originally reticulated. The majority of this surfacedecoration has been lost to erosion. The pits of the reticulation, as wellas all other sheltered parts of the archway have a sulphate crust, which isblistering and contributing to the decay of the masonry. The main bodyof the archway is built of the same limestone, in well-coursed dressedrubble.

The upper surface of the pediment has suffered extensive frost damage;large sections have delaminated; there are significant water-traps whichwill channel rain and snow melt into the body of the pediment and arch.

Pediment cap, lower side

There is extensive plant growth and consequent accumulation of organicmaterial in the joints of the pediment top.

There is ongoing delamination of the faces of the arch stones. Onremoval of loose flakes, salt probably gypsum/calcium sulphate crystalsare clearly visible behind.

salt-induced delamination, keystones

Salts may be migrating from the top of the pediment, and also from theretained soil around and behind the arch into the stonework. However,the likely cause of gypsum-induced decay is simply that the fabric of thearch is too wet. The pediment roof has been letting in water over a longperiod and opc mortars have prevented its ready egress through thejoints, leading to a concentration of moisture in the stones of the archthemselves. The use of cement-based mortar introduces damaging saltsinto the masonry substrate in and of itself. Ordinary Portland Cement is30% sulphate.

hard opc mortar in pediment joints

The intended breatheability of the arch has been compromised over theyears by the use of opc mortars. There are two periods of cement-basedrepointing. The most recent is of a relatively soft opc mortar, much of itlaid over otherwise sound lime mortar. It is easily removed. There is someearlier repointing, however, which is very hard and based upon whitecement. White cement sets harder than grey. This mortar has beenpressed deeply into the joints. Around these joints there has been a buildup of leeched out material, possibly previously uncarbonated lime whichhas set in solution with other compounds from the white cement. It ishard and unsightly. Its removal is necessary, but it has damagedadjoining arrisses.

The construction of the arch is of a high quality, although the vanity ofeither the architect, the masons or both meant that it was builtthroughout with very tight joints. In its specific location, the practicalconsequence of this has been to limit the sacrificial function of the limemortar joints, significantly reducing the surface area of lime joints for thepurposes of rapid evaporation of moisture. The pore structure of thisparticular limestone is relatively dense, making it less able toaccommodate the freezing of water or the crystallisation of salts withinthat pore structure without damage. The soft evaporation surface hasbeen even more reduced by the addition of opc mortar repointing, so that

excessive evaporation has taken place from the stone itself, leading to aprogressive spalling of over-moist stone faces. Many of the stones have aconcave profile as a result, the stone adjacent to the joints having faredrather better.

Removal of cement pointing shows fineness of joints

A case may be made for the gentle removal of sulphate skins. This wouldinvolve the use of micro-abrasive under low pressure in the hands of anexperienced stone conservator. Prior poulticing of the sulphate skins withsaturated paper pulp would facilitate easy removal. Sand-blasting is NOTan option. Such a procedure is not part of the scope of these works,although the loose edges of blistering will be defrassed. Sulphation oflimestone locally will be an ongoing problem so long as HGV-use of theseroads is high.

All cement mortars were removed, all loose flakes of stone also. Fournumber losse stones to the pediment cap were loose. These wereremoved and rebedded. The two stones to the front were originally partof one stone that has been broken into three pieces in the past.

The joints were repointed using a lime mortar; the joints were thoroughlycleared of organic matter and repointed with lime mortar. All water trapswere filled with lime mortar, including the tops of projecting key-stones,to provide weatherings. Large areas of delamination to the pediment topwere built up with a stone-dust rich lime mortar, to compliment thecolour of the original stone. The arch was gently cleaned down using softbrushes and water before, during and after the repointing process. Therepair--as opposed to the repointing-mix on the pediment was 1hl2: 1limestone dust (5mm to dust): 1 sharp sand: 1 plastering sand. Themortar repairs to the pediment top consisted of : 5 hl2: 6 limestone dust:3 sharp sand: 3 plastering sand. The finer joints of the main structurewere pointed with 1hl2: 1 limestone dust: 2 plastering sand. Theperformance of these mortars over the winter will be assessed in the

spring for vulnerability to frost damage.

The overall character of this arch is one of relatively graceful decay underthe influence of moisture, frost, salt attack, and wind erosion. No attemptshould be made to restore this arch; presumption should be made infavour of preserving its present character and of managed decay.

the earth stone and lime companybuilding conservation consultancy and practice

LIME MORTAR

The chemical components of an ordinary portland cement and a naturalhydraulic lime may often be common. What becomes active in op cement,remains inert in an NHL. This can lead to confusion and uncertainty insome laboratory testing methods.

Opc mortars, such as a 1:1:6 cement: lime: sand mix introducepotentially damaging sulphates into masonry walls--in the presence ofmoisture these will be mobilised and their crystallisation may lead to theaccelerated erosion of arrisses and even the faces of softer, but also ofharder stones. The addition of even small quantities of opc (or whitecement) will prevent any carbonation of free lime in the mortar. Thismeans that the lime content is a plasticiser and an aggregate only. Itdelivers no additional flexibility to the finished mortar.

A lime mortar introduces no salts.

A cement mortar, because of its flash set, is riddled with microscopic(and often quite visible) shrinkage cracks from day one--capillary actionwill pull moisture deep into the structure; water will be trapped and leavethe fabric more vulnerable to frost action.

Water accumulates and moves unpredictably through masonry structures.

Lime mortars do not rely upon the resistance of water for their success-They accommodate the ingress of moisture, and then allow its rapidevaporation via the mortar joints, not the face of the stone or brick asoccurs when these are pointed with a hard cement mortar. Lime does notallow moisture to linger in the fabric and minimises the potential for frostand soluble salt damage. It is eminently vapour permeable; it offersmaximum breatheability.

Lime renders act as a sponge on the face of a building, soaking up waterduring rainfall, allowing its steady evaporation once the rain has ceased.It is a sacrificial application, that will degrade slowly over time, to be

repaired and replaced. Its application is relatively inexpensive in materialsand labour

Lime mortars are flexible, allowing often quite major settlement to occurin solid wall constructions without compromising their structuralintegrity. Minor movement will lead to cracking in rigid cement mortars,and to the build up of stresses that lead ultimately to spalling andcracking of stones as well as of mortar. Hairline fractures in lime mortarsare ‘self-healing’ being filled over time by leeched calcium carbonate insolution from the mortar.

The character of a traditional masonry structure is given as much by itslime mortar as by its stone. There is a unity between the mortar and thestone; its form of construction was moulded by the combination.

Any serious attempt to recapture the essential character of a structure-itsessence- will involve the use of similar, traditional materials laid up withempathy for and understanding of the methods and procedures imposedupon the mason by these materials and their combination. The use ofmore modern materials will compromise not only the ability of atraditional building to function as its builders intended, but will radicallycompromise its aesthetic appearance.

Sand used for lime mortar should be well-graded, having a good range ofparticle size; coarse/sharp or well-graded plastering sand is appropriate,according to the fineness of the masonry; the addition of well-gradedcrushed stone is appropriate; the use of Portland cement in anyproportion is wrong-headed and inappropriate as well as unnecessary inany situation.

St. Astier Natural Hydraulic Lime is readily available ; hl2 hydraulic limefrom Somerset, England also. Both are reliable products that delivergood performance and all the advantages of a lime mortar. They come inthe form of a bagged hydrate, and may be mixed in the same way as acement. It is greater by volume than cement; 25kg will go up to 3 timesas far.

St Astier NHL 3.5 or 5.0 lime should not be used on soft of decayed limeor sandstones. Being well-enough suited to use with granite or otherigneous stones, or for new-build with harder limestones and sandstones,

it delivers too hard a mortar in my opinion for use with softer stones.Putty lime, St Astier 2.0 or HL2 hydraulic lime from the UK, with hybridmixes of NHL and putty lime not ruled out, are more appropriate for usewith historic lime and sandstone masonry, delivering less compressivestrength and greater vapour permeability.

The use of bagged, hydrated lime available from most builders merchantsshould not be automatically ruled out where cost-pressure wouldotherwise lead to the use of ordinary Portland cement.Whilst historic non-hydraulic lime mortars were made with limestoneburned at relatively low temperatures, leaving many impurities availablefor later chemical reaction, this is not the case in the production of themajority of quicklime used to produce lime putty today. Modernquicklime is produced at very high temperature, producing a very pureproduct. This means that there is very little chemical difference betweenbagged and putty lime. The main difference is in the maturation time ofthe rehydrated lime. The greater maturity of putty lime delivers a greaterplasticity in use and arguably a more reliable product. The plasticity ofbagged lime may be increased by slaking it in water before incorporatingit into a mortar. Research by Norman Weiss at Columbia University’sDepartment of Historic Preservation suggests that the optimum period forthis is 48 hours, no increase in plasticity being derived thereafter.

If bagged lime is used, it should be used in conjunction with a pozzalanto assist the set. This could be Metastar (calcined china clay); appropriatebrick dust or trass. Bagged lime may also be gauged with a naturalhydraulic lime, both to assist speed of set and to strengthen the mortar.

PhD research into the performance of a range of limes and limes incombination for use in pointing and mortar repair is currently beingcarried out at Bath University by Mike Lawrence, owner of the Ham HillStone company.

The successful use of lime requires careful preparation ofsubstrates/stones; sensitive application and conscientious aftercare, yetthere is no mystery to this.

BEDDING

Stones or bricks should be well-wetted (preferably submerged indrinkable water and only removed shortly before use) before being builtinto the wall. The mortar should be quite stiff, not at all sloppy; quiteunlike a standard, modern brick-laying mix. The joints may be strucksimultaneously with the stone-setting, but the aftercare will be the same.The aggregate must be ‘brought-up’ to enhance the evaporation surface,as well as the appearance of the masonry. This is best achieved bybeating the mortar with the bristles of a churn brush, using short, sharpblows. This simultaneously consolidates the mortar. Other methods, suchas ragging and brushing risk pulling the mortar away from the substrate.

When repointing brickwork, it is essential that the substrate be well-wetted. However, it is very important that the brick is allowed to dry somebefore any mortar is introduced. The mortar should be as dry as ismanageable/workable, to minimise any running of wet mortar or lime-rich water from the mortar, onto the face of the brickwork. It is very easyto stain brickwork during repointing work, and difficult to remove thisstaining once the lime has carbonated.

POINTING

Joints should be raked back at least two times their width, leaving anyshims of stone introduced into wider joints by the original builders inplace if at all possible (setting them aside for reuse, if they come loose).

The use of angle-grinders during the mortar removal process should notbe allowed. Mechanical blades are too fast, too aggressive to enable theconsidered removal of defective mortars. Shims of stone within joints willbe destroyed, and thus will the character of the masonry, as well as

historic fabric be compromised.In even the most skilled hands, angle grinders will cut into the brick orstone of the building, leaving unsightly scars. The sharpness of the arrissthat may be created in otherwise rough stone ashlar can change theoverall character of a masonry elevation significantly.

All dust and debris should be flushed out of the joints with water andareas to be repointed should be saturated twenty-four hours beforebeing worked upon using hoses.

The process should be repeated immediately prior to the start ofrepointing work and the hose played regularly upon lower areas as yetunpointed even whilst repointing is being carried out above.

The necessity for copious pre-wetting cannot be underestimated.

The drying action of the older mortar inside the wall, as well as of theporous brick or stone would otherwise compromise the adhesion of thenew mortar.

Consideration must be given, however, to the possibility or likelihood ofwater penetrating to the interior of the building, causing previouslysound iron fixings to corrode, the mobilization of soluble salts to thedetriment of interior finishes, or other associated damage.

The new mortar should be mixed thoroughly prior to use.

Natural Hydraulic Lime-based mortar and hydrated masons lime will workbetter if re-worked in the mixer 24 hours after first being mixed,although no further water should be added at this stage. This is notessential, and requires experienced judgement. When using the mortardirectly after mixing it, workability of the mortar is enhanced by initiallymixing the lime only with the water, the aggregate being then added tothe lime slurry until the mortar becomes sufficiently stiff.

Putty lime mortar will be easily worked and as plastic immediately afterfirst mixing.

Joints should be wetted again before pointing, but should not beglistening wet when pointing begins.

The mortar should be sticky and stiff, not over-wet or sloppy.

It should be forced into the joint with a pointing iron narrower than thejoint width, or fed into the joint with an iron from a trowel or hawk with aminimum of affectation.

It should be compressed but not over-worked, as this would bring toomuch lime to the front, leaving the main body of the mortar lime-leanand weak. The joint should be left over-full and ‘untidy’ at this stage.

The newly pointed areas should be hung with doubled burlap, whichshould be damp and which may be regularly dampened thereafter,although not so much at this stage that the mortar behind begins todissolve and run down the face of the masonry.

The mortar derives its strength from having within it a good range ofaggregates and from the steady progress of the hydraulic set and thesubsequent carbonation of free lime in the presence of moisture. Thewalls should be kept covered and moist for at least a week, preferablytwo, and protected from sun, wind and driving rain as well as from frost.

It is quite possible to work with, particularly hydraulic, lime mortarsduring the winter months. However, careful attention must be paid toprevailing and predicted weather patterns, and appropriate action takento minimise the risk of losing mortar to frost. All chemical activity in alime mortar ceases below 5 degrees Centigrade, and work should not becarried out if temperatures are likely to drop below 10 degrees C for anylength of time. Prolonged cold and wet weather will seriously compromisethe setting of putty lime mortars and leave them highly vulnerable tofrost action. As a general rule, these should not be used December toApril. Any work with lime mortar carried out in winter should be coveredwith hessian for as long as possible after completion of the work.

Surplus mortar may be raked off with a piece of wood after a few hours,or after 24 hours, depending on the weather conditions and the predictedsetting time of the lime used, when to do so will not cause any smearingof the mortar onto the face of the masonry.

The mortar should be ‘beaten back’ with a stiff brush around 12-36hours after being applied, at a point when it has attained a stiffness thatdoes not allow ‘pin-holes’ to be made by striking bristles or lime to belifted by the bristles and redistributed onto the stones around.

It should not be so stiff as not to allow the removal of lime bloom and theaggregate to ‘come up’ on the beating of the mortar. Leaving a mortar‘off the trowel’ leaves a lime rich and therefore harder and less

permeable layer which will reduce the breatheability of the mortar andlead, over time, to the decay of the mortar immediately behind the face.

The mortar should then be re-covered. The regular, but consideredwatering of the wall hereafter will promote the entrainment of carbondioxide deeper within the structure of the mortar and help to maintainand to prolong a steady carbonation of the lime.

It is important to stress that a failure to follow this basic procedure willlead to an immediate or premature failure of the mortar.

A properly designed lime mortar, correctly applied, will perform itsintended function for many years and may be expected to last for morethan a century. This is something that may not be said of an ordinaryPortland cement-based mortar, which will rarely last more than 20 yearsin the context of a traditional building and which is very damaging to thefabric in the meantime. Any contractor using lime that does not pre-wetthe masonry, and does not protect the mortar from too rapid drying isnot competent to be carrying out the work.

When the mortar has gained its fuller initial set, after 2 or 3 weeks, thenthe wall as a whole should be brushed down with a stiff brush and anylime staining teased from the stone with fine stainless steel brushes asnecessary.

VOID TESTS

Void tests establish the amount of lime required to coat each grain ofaggregate in a particular sand. They are performed by adding water to ameasured quantity of dry sand until the sample is covered with water.The amount of water added is then calculated and an optimum sand tolime proportion derived.

Useful websites:

www.spab.org.uk

www.buildingconservation.co.uksite of the Building Conservation Directory, details of specialistcontractors, articles.

www.historic-scotland.gov.uk

www.limesolve.demon.co.ukwebsite of hl2, Hydraulic Lias Limes Ltd, UK.

www.nigelcopsey.com

www.donhead.com

Useful Publications:

Survey and Repair of Traditional Buildings, a Sustainable Approach,Richard Oxley, Donhead;

Historic Scotland Technical Advice Note 1 : Preparation and Use of LimeMortars;

The Repair of Historic Buildings in Scotland,

Technical Advice Note 10, External Lime Coatings on TraditionalBuildings, Historic Scotland.

Available from: Historic Scotland, Scottish Conservation Bureau,Longmore House, Salisbury Place, Edinburgh, EH9 1SH, Tel.: 011 44 131668 8668.

Repair of Ancient Buildings, AR Powys (available from SPAB, 37 SpitalSquare, London, E1 6DY, Tel.: 44 208 247 5996)

Structural Repair of Traditional Buildings, Patrick Robson, available fromDonhead Publishing Ltd, Lower Coombe, Donhead St Mary, Shaftesbury,

Dorset, SP7 9LY.

Cleaning Historic Buildings (2 volumes), Nicola Ashurst, Donhead Press,Shaftesbury, Dorset;

Conservation of Building and Decorative Stone, Ashurst and Dimes,Butterworth Heinemann;

Society for the Protection of Ancient Buildings Technical Pamphlets:

www.spab.org.uk

The Need for Old Buildings to Breathe, Philip Hughes;

Pointing Stone and Brick, Gilbert Williams;

Treatment of Damp in Old Buildings, Andrew Thomas;

An Introduction to Building Limes, Michael Wingate;

Basic Limewash, Jane Schofield.

SUPPLIERS:

Hl2 lime:

Hydraulic Lias Limes Ltd, Tout Quarry, Charlton Adam, Somerset, TA117AN, United Kingdom. Available from Walkers builders merchants, FossIslands Road, York.

Putty lime; limestone dust; premixed plasters: Ryedale Conservation,Terrington, North Yorkshire.

St Astier Natural Hydraulic Limes available from Womersley Associates,Heckmondwike, West Yorkshire. St Astier 5.0 is available from Walkers,York.

Stone dust: 5mm to dust from Wath quarry, Hovingham, North Yorkshire,owned by Lafarge Aggregates.

SUGGESTED LIME MORTAR DESIGNS FOR USE IN THE MALTON AREA:

These mortar designs have been arrived at after assessment of historicmortars found in Malton. They provide a basis only. There is no objectivereason to slavishly reproduce a mortar to an historic design that mayhave failed. However, attention should be paid to historic mortars asfound on a project by project basis.

For bedding and repointing of rubble walls and decayed, erodedstonework:

1 part hl2; 1 part limestone dust; 2 parts sharp sand;

OR 1 part putty lime (+10% by volume of lime, Metastar calcinedChina clay; aggregates as above;

For finer ashlar:

1 part hl2; 1 part limestone dust; 2 parts plastering sand;

OR substitute 1 part putty lime + 10% metastar;

For brickwork:

1 part hl2; 1 part limestone dust; 1 part plastering sand; 1part sharp sand;

Or: 1 part hl2; 1 part limestone dust; 2 parts sharp sand;

Or: 1 part hl2; 1 part sieved limestone dust; 2 parts plasteringSand.

Or: substitute putty lime (+metastar) for hl2.

The choice of mortar for brickwork will depend upon the type of bricksused, whether 2” C17 bricks or clamp bricks, wire cut or machine-madeVictorian bricks. The type of brick used determines the optimum thethickness of the joint.

Mortar repair and mixes will incorporate stone dusts to a possibly higherproportion in pursuit of a colour compatible with the stone substrate. Iam in the process of trialing some mixes using Ancaster, bath stone andham stone dusts, as well as sandstone dusts used locally in themanufacture of pre-cast stone. Crushed sandstone from the Blue BankQuarry in Sleights may offer similar possibilities.

Long-term, there would clearly be great advantage in the location,identification and re-opening where possible of former quarries locally.Such proposals tend to fall foul of local and sometimes statutoryopposition, and the suggestion by Jefferson Consulting that any re-use ofold quarries should be for the specific purpose of repair and conservationworks (and possibly for limited new use in the context of brownfieldenabling developments where appropriate) and should use low-impactmethods of extraction to minimise environmental impact is sound.

There are a number of abandoned quarries in the Malton area. The onesclosest to the town have been adopted for other uses. There is a largeabandoned quarry on York Road, however, that has found no other use,and which is now woodland and scrub. The quarry is large and probablysupplied a large volume of the sandstone used locally. The stone wastransported from the quarry to the River Derwent via a tunnel beneathYork Road, which tunnel remains, albeit blocked.

The quarry-faces are up to 12 metres high in places, with bedding planesclearly visible. It would seem that the seams of stone were by no meansexhausted when the quarry closed. The trees growing on the quarry floorare of a good age, 100 years at least, being many of them Beech.

This quarry would seem to be the most likely viable source of like stonefor use in conservation and repair (and possibly new build) works inMalton.

York Road sandstone quarry.